Striking The Ground in Sprinting

Striking The Ground in Sprinting

QUESTION

I read somewhere that a major difference between people who are considered fast and those who aren’t lies in technique. It was written there that some of the people when sprinting let their foot fall passively to the ground, and then at initial contact they try to push the ground resulting in too late and too weak of force to the ground. Whereas other people, those who are considered fast, hammer the ground and start pushing down hard when the knee is at its highest when the foot is still in the air, and after initial contact they already start to pull their leg forward. A hammer and a nail analogy was given to describe this, saying that it is impossible to produce high force when the hammer is resting on the nail, in contrast to if its being pushed down from above… Sprinters produce force to the ground of 5 times their body weight, I can’t see how that’s possible without hammering down your foot. What do you think about this whole argument?

ANSWER

Your body is not a hammer. And a hammer resting on a nail is an inaccurate analogy.

Each stride has a small vertical displacement, so there is some downward momentum of the whole mass of the body. The high forces come from a very fast stiffening of the leg upon planting to stop and reverse that downward momentum, not from slamming the foot into the ground really hard. That might increase the initial impact force, but it interferes with the multi-joint stiffening and the total impulse of the plant.

We can see this more clearly on depth jumps. If people pick up their feet and try to drive them down into the ground to add to the GRF, it ruins the jump. Again, the high GRF should come from the mass of the whole body dropping toward the ground and a fast, stiff plant reversing that momentum.

It’s also observable during early acceleration. Exaggerating knee and foot lift and giving the foot more downward momentum at the ground does not produce a quality contact. Again it’s a quick, stiff plant that we want, not a stomp.

The purpose of picking up the feet and attacking the track is actually to generate backward foot velocity to minimize braking force. And faster sprinters do tend to do this better. But the ability to do it better requires getting off the ground more quickly, so it all comes back to vertical force production. (See my speed science series)